This invention addresses the determination of the range parameter in a scannerless range imaging system. In general, such a system functions by sending modulated radiation (preferably optical radiation, but radio or sonics may also be used) toward a target area. An object in that target area will reflect the radiation, resulting in a reflected signal which can be detected at either the source of the original radiation or at a distinct, but known, site. Because the radiation has traveled the distance corresponding to the path connecting the source, object, and detector, the detected radiation will exhibit a phase shift relative to a reference signal derived from the source of the original radiation. Given a knowledge of the geometric, optical, and electronic time delays which characterize the scannerless range imaging system, comparison of the received signal with the original signal allows the phase shift, and hence the distance to the object, to be determined.
Imaging of the range information to obtain three-dimensional information about the distance and shape of an object has traditionally been carried out by scanning the radiation source (e.g., a focused laser beam) in a raster-like manner across the object of interest, and measuring the phase shift for each point on the object. This is a time-consuming process, primarily because of the weight of the scanner optics compatible with a fast, large aperture scanner. The absence of suitable scanners prevents use of laser radars in many potential applications.
The need for a scanner can be eliminated by using a detector array on which the entire scene is imaged simultaneously. Typical of such detector arrays are CCD arrays, which are available with hundreds or even thousands of pixels on a side. They convert incoming photons into electrical charge with reasonably high efficiency (generally more than 20%), which is stored within the detector element until read out. Such detectors are suitable for forming an intensity image. However, as they integrate the incoming light, they are not suitable for direct determination of the phase shift of the modulation of the reflected signal.
Scannerless range imaging systems do exist in the prior art, in particular in U.S. Pat. No. 4,935,616 (Scott), which is included herein in its entirety, and in art referred to therein. The primary focus of the present invention is to improve the invention of Scott.
Scott developed a scannerless range imaging system in which the phase shift information is extracted by beating the reflected signal against a reference signal which is phase-locked to the illuminating laser signal. Briefly, this beating is accomplished by directing the reflected signal into an image intensifier whose gain is periodically varied by an external control voltage. The integral signal output of each CCD pixel then contains information about the path phase shift from the illumination source to the point on the object being imaged on that pixel to the detector pixel.
Scott describes two approaches to deconvoluting the phase information from a series of intensity measurements taken using additional phase shifts between the illumination source and the image intensifier gain. Both approaches depend on modulating the voltage between the photocathode and the thin conductive sheet so that the overall gain function of the image intensifier (the unit comprising Scott's photocathode 34, thin conductive sheet 38, microchannel plate 44, and phosphor plate 46) has the same functional form as does the modulation of the output beam 16. However, if an appropriate sinusoidal voltage is simply applied to a nonlinear image intensifier, Scott's deconvolution procedures do not function satisfactorily, owing to the significantly nonlinear gain vs voltage dependence of commercial image intensifiers. Thus, to implement Scott's invention as disclosed requires that a complex nonsinusoidal control voltage be generated and used to control the gain of the image intensifier.
Scott's claims and specification teach an operable scannerless range imaging system. However, considerable simplification of a practical Scott-type scannerless range imaging system would result if a (for example) simple sinusoidal control voltage can be used to control the image intensifier gain. There is thus a need for a method of operating a scannerless range imaging system comprising nonlinear components which allows the use of sinusoidal (or other simple periodic) control voltages while avoiding the analytical and/or circuit design complications required to use the range determination procedures taught by Scott.